Large cities may have thousands of miles of 3-phase, high-voltage, underground electrical conductors in their electrical power network. Electrical equipment such as transformers and switch-gear used to manage and maintain the flow of electricity through these conductors is typically housed in underground vaults. Numerous junction points connecting the underground electrical conductors are often accessible through manholes in the city streets. Boston, Mass., as an example, has over 100,000 manholes to provide access to its underground electrical network.
A single underground location will often have multiple, possibly dozens, of electrical conductors passing through it en route to various destinations. Multiple conductors going to the same destination often result from the inherent limitation in the electrical capacity of a single phase electrical conductor. Phase refers to a differences between the zero crossing points of several simultaneous transmissions of alternating electric current that are made on electrical current conductors, that is, each zero crossing point is delayed by a phase angle from the zero crossing points of the other transmissions of the alternating current traveling on different electrical conductors that comprise the three phase transmission. Conductors and other electrical equipment have the same phase when they are electrically connected so that alternating current can be sent from one conductor to any other in phase with it.
Moreover, because of the need to arrange conductors between junctions so that local customers may be served, multiple conductors are used to and from each junction. For example, a location accessible by manhole could contain multiple, parallel conductors meeting at several junctions, each of which is attached to additional conductors at other junctions, and others running from there to junctions at other locations. These other conductors could serve two, three, four or more additional locations that are miles from each other in the geographical area of the power network.
A power outage may be caused by a defective conductor or a failure of another electrical system component. To resolve the outage, It may be necessary to isolate the affected portion of the electric power grid in order to repair or replace those failed components. The term isolated means to be removed from electrically conductive contact from an electrical source and an electrical load, that is, disconnected from the source of electrical power and from the connections to local users of that electrical power. Standard electrical system practice dictates that, after isolating and identifying the section of conductor that is not transmitting electrical power, the conductor that has been removed from service must be electrically grounded. This practice requires that the conductors carrying three phases of the alternating current transmission must each be electrically grounded with grounding jumper conductors at the source, at each of the end points, and at any intermediate junction points that are accessible. Electrically grounding a conductor has a specific well-defined meaning provided by ASTM F855, a standard promulgated by the American Standards for Testing and Materials. This ASTM has a rating for a 4/0-5H grounding jumper conductor that may withstand 126,000 peak amps and 47,000 RMS amps for 15 cycles at 60 Hz.
Applying or removing grounding jumper conductors from these electrical conductors when they have been de-energized and removed from service can be a major undertaking involving the coordination of dozens of employees including system control supervisors, power dispatchers and many field people. In addition, grounding jumper conductors are physically large, heavy and cumbersome to remove and install. A grounded three-phase conductor may remain out of service and electrically grounded for an indefinite period of time. This concerted action may take many hours depending on the complexity and routing of the conductors involved. If the safety grounds are removed from the wrong conductors, time is wasted and, more importantly, an additional safety hazard is introduced.
There has been no method prescribed and no equipment available to the electrical utility industry for testing to determine the proper phase sequence of a three-phase conductor that is electrically grounded, and no way to determine the phase sequence of grounded conductors before removing the grounding jumper conductors. In fact, because signal tracing equipment works on electrically ungrounded conductors, it is industry practice to first remove the grounding jumper conductors and then identify each piece of equipment that is in phase with each other piece of equipment, which is a major reason the repair of a portion of an electrical grid requires so much time and so many employees. In addition, the action of removing the grounds introduces a safety hazard to those employees testing the conductors from unwanted stray induced voltages on the de-energized conductors by nearby energized conductors and from the inadvertent energizing the conductor undergoing testing.
A method for testing and determining the phase of electrically-grounded, three-phase conductors before removing the grounding jumper conductors would have significant advantages to electrical utilities in safety and time saved, and to the customers they serve.